Disease Risk & Landscape Attributes of Tick-Borne Borrelia Pathogens in the San Francisco Bay Area, California.

Daniel J Salkeld,Stephanie S Cinkovich,Michael Carbajales-Dale,Patricia Carbajales-Dale,Nathan C Nieto,Eric F Lambin,Brian Stevenson

doi:10.1371/journal.pone.0134812

Daniel J Salkeld, Stephanie S Cinkovich + Show 5 more

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https://doi.org/10.1371/journal.pone.0134812

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Abstract

Habitat heterogeneity influences pathogen ecology by affecting vector abundance and the reservoir host communities. We investigated spatial patterns of disease risk for two human pathogens in the Borrelia genus–B. burgdorferi and B. miyamotoi–that are transmitted by the western black-legged tick, Ixodes pacificus. We collected ticks (349 nymphs, 273 adults) at 20 sites in the San Francisco Bay Area, California, USA. Tick abundance, pathogen prevalence and density of infected nymphs varied widely across sites and habitat type, though nymphal western black-legged ticks were more frequently found, and were more abundant in coast live oak forest and desert/semi-desert scrub (dominated by California sagebrush) habitats. We observed Borrelia infections in ticks at all sites where we able to collect >10 ticks. The recently recognized human pathogen, B. miyamotoi, was observed at a higher prevalence (13/349 nymphs = 3.7%, 95% CI = 2.0–6.3; 5/273 adults = 1.8%, 95% CI = 0.6–4.2) than recent studies from nearby locations (Alameda County, east of the San Francisco Bay), demonstrating that tick-borne disease risk and ecology can vary substantially at small geographic scales, with consequences for public health and disease diagnosis.

Highlights

Human infection by tick-borne pathogens is the culmination of interactions between the transmission biology of the pathogen, the ecology of the reservoir hosts and competent vectors, and the consequent exposure and disease in the human case
We report three measures of entomologic disease risk: (i) Density of nymphs (DON)—i.e., the number of nymphal ticks found per 100m of transect, for both 2012 and 2013; (ii) Infection prevalence of Borreliae, i.e., percentage of ticks testing positive for the disease agent for samples from 2013; (iii) Density of infected nymphs (DIN), calculated by multiplying nymphal infection prevalence by the density of nymphs, and reported as the density of infected nymphs/ 100m
In Thornewood Open Space Preserve (OSP), DON was higher (3.1/100m2) in coast live oak forest compared to redwood habitats (1.2/100m2)

Summary

Introduction

Human infection by tick-borne pathogens is the culmination of interactions between the transmission biology of the pathogen, the ecology of the reservoir hosts and competent vectors, and the consequent exposure and disease in the human case. Tick-Borne Diseases in California conditions can influence any and all aspects of these interactions, and disease risk and incidence vary as a result [1,2]. California exhibits a high degree of climatic and habitat heterogeneity which influences tick abundance, reservoir host communities, and the entomologic risk of Lyme disease [1,2]. Hardwood-dominated woodlands exhibit higher densities of nymphal ticks infected by Borrelia burgdorferi, the etiologic agent of Lyme disease, than do conifer-dominated woodlands that include redwood or pine [2]. Human incidence varies within the state, with most cases occurring in northwestern counties, and fewer cases being reported from southern California [3]

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